1. Field of the Invention
The present invention relates generally to reactivity control systems for nuclear reactors and, more particularly, is concerned with a self-actuated nuclear reactor shutdown system employing an induction pump for drawing a portion of coolant flowing from the exit of adjacent fuel assemblies into the shutdown system so that the coolant temperature can be sensed directly.
2. Description of the Prior Art
Typically, a liquid metal reactor (LMR) has a core which contains a multiplicity of nuclear fuel rods disposed in a plurality of fuel assemblies for generating a nuclear reaction which heats a coolant flowing through the fuel assemblies. Also, the LMR includes a number of neutron absorber rods disposed in two basic types of control rod assemblies for regulating core reactivity: a primary control rod assembly and a secondary control rod assembly. The three types of assemblies, each being preferably hexagonal in cross section, are disposed adjacent to one another in a predetermined arrangement in the core.
A primary control system operates each primary control rod assembly to maintain reactor criticality by adjusting the quantity of neutron absorber or poison material (typically boron carbide) in the active core region and to effectuate shutdown in normal and abnormal operations. Typically, the absorber material (often contained in rod bundles) of each primary control assembly is reciprocated vertically within the core between several adjacent fuel assemblies to control reactivity.
A secondary control system operates each secondary control rod assembly as a backup to the primary control system shutdown function. The absorber rod bundle of each secondary control assembly, commonly referred to as a shutdown assembly, is ordinarily held in a raised position outside the core region and is only released and dropped into the core for shutdown purposes, if the primary system fails.
Generally, both types of control systems are activated manually (through operator action) or by various sensors which monitor the core conditions. In each case, external circuitry is used in the shutdown process. In order to provide another level of reactor protection, systems have been designed which eliminate the need to rely on external circuitry to sense and effect shutdown. One example of such systems is a self-actuated shutdown system for each of the secondary control assemblies as disclosed in U.S. Pat. No. 4,304,632 issued to S. K. Bhate et al and assigned to the same assignee as the present invention. This shutdown system employs a temperature sensitive magnetic latch to support the bundle of absorber rods of the respective secondary control rod assembly outside of the active core. The material that caries the magnetic flux at normal operating coolant temperatures, is designed to loose its magnetic capabilities at elevated temperatures, such as would develop during an accident event. The magnetic latch cannot support the weight of the absorber rod bundle at these elevated temperatures, and, consequently, will drop the bundle into the core if the primary and secondary control systems fail.
However, the typical absorber rod bundle generally operates at coolant temperatures significantly below the average coolant temperature of the core (the coolant temperature of the fuel assemblies being much higher than that of the control assemblies) and during a transient over-temperature event the coolant temperature response of the absorber rod bundle is much slower than the average core coolant temperature. In order to maximize the speed and reliability of the self-actuated shutdown system, the above-cited patent used a number of dedicated fuel rods installed between the inner and outer ducts in the absorber rod assembly to simulate the higher coolant temperature in adjacent fuel assemblies. This high temperature coolant was then used at the magnetic latch location to actuate the shutdown system. Specifically, the temperature sensitive component of the magnetic latch, being positioned on a fully withdrawn secondary control rod assembly, would sense the temperature of the outlet coolant of these dedicated shutdown assembly fuel rods, which would be typical of fuel assembly coolant temperatures, and respond quickly to the rising temperature of an accident event. These dedicated fuel rods also provided the flexibility to tailor the self-actuated system to a particular core design by controlling the fuel rod coolant flow.
Norwithstanding the overall satisfactory performance of the above shutdown system, several disadvantges have been found to reside in this particular design. First, the cost of manufacturing these self-actuated shutdown control assemblies is much higher than the standard control assemblies due to the added complexity of including the dedicated fuel rods. Second, the combination of fuel and absorber material in the same unit results in more costly disassembly procedures when the assembly is removed from service. Third, the use of the dedicated fuel rods and the fixed position of the temperature sensitive material requires that the shutdown assembly operate only in a fully withdrawn position and thus the assembly cannot be used as a primary control rod assembly. Finally, the dedicated fuel rods reduce the space available within the shutdown assembly for absorber material. Consequently, a need still exists for a reactivity control rod assembly which will reduce the complexity and cost and improve the flexibility of implementing a self-actuated shutdown system while retaining the benefits to be derived therefrom.